1. Field of the Invention
The present invention relates to a constant current circuit and a voltage reference circuit using the same, and more specifically, to a constant current circuit which can maintain operation in a weak-inversion state even if a junction current flowing between a drain and a substrate and between a source and the substrate occurs at high temperatures.
2. Description of the Related Art
The following describes a conventional constant current circuit. FIG. 6 is a circuit diagram of the conventional constant current circuit. The conventional constant current circuit is constituted by enhancement N-channel transistors 61 and 62 having different K values, enhancement P-channel transistors 63 and 64, a resistor 65, an earth terminal 100, and a power supply terminal 101. A K value is obtained according to K=W/L·(μCox/2), where W denotes a gate width of a transistor, L denotes a gate length of the transistor, μ denotes a mobility of a carrier, and Cox denotes a gate oxide capacitance per unit area.
In the enhancement N-channel transistor 61, its source is connected to the earth terminal 100, and its drain and gate are connected to a gate of the enhancement N-channel transistor 62 and a drain of the enhancement P-channel transistor 63. In the enhancement N-channel transistor 62, its source is connected to the earth terminal 100 via the resistor 65, and its drain is connected to a gate and a drain of the enhancement P-channel transistor 64 and a gate of the enhancement P-channel transistor 63. Sources of the enhancement P-channel transistors 63 and 64 are both connected to the power supply terminal 101.
A K value of the enhancement N-channel transistor 61 is smaller than a K value of the enhancement N-channel transistor 62. A gate-to-source voltage difference between the enhancement N-channel transistor 61 and the enhancement N-channel transistor 62 occurs in the resistor 65, and a current flowing in the resistor 65 is mirrored by the enhancement P-channel transistors 63 and 64 so as to generate a bias current.
[Patent Document 1] Japanese Patent Application Laid-Open No. 3-238513 (FIG. 4 (a))